This invention relates to multi-layer flexible printed circuit board assemblies. The invention will be described with reference to an example where the assembly is effectively installed in a single-lens reflex camera.
Recently, electronic components have been extensively employed for use as exposure adjustment mechanism in single-lens reflex cameras. Instead of a CdS cell, an ammeter and variable resistor employed in a single-lens reflex camera with a through-the-lens (TTL) type exposure meter, electronic semiconductor components such as transistors have been extensively employed as a different type of exposure system. Accordingly, the circuit becomes rather intricate, and the number of components forming the circuit is increased. In view of workability and reliability, it is undesirable to connect the electronic components with lead wires. Also, the space considerations in such cameras limit the area available for such circuitry. In order to solve these problems, printed circuit boards have been proposed.
Circuit boards have been employed for wiring in a camera since the earliest single-lens reflex cameras, although the circuit itself it of simple construction. At that time, a hard printed circuit board called "a rigid printed circuit board" was used. Thereafter, the number of electronic components installed in a camera has steadily increased, and accordingly the space requirements dictate economies of use for a given small space in a camera body. Thus, flexible printed circuit boards which can be relatively freely installed in a camera have been employed.
Electronic components in a camera body, such as a light receiving element, a display element, a calculation element, and a control element are often coupled to the flexible printed circuit board. It has been the recent tendency for installation of electronic components in the camera, because the work efficiency and reliability are improved by using a flexible printed circuit board. However, as camera design becomes more complex and the number of functions performed has increased, the number of electronic components to be installed therein has accordingly increased and the wiring on the printed circuit board has become more intricate.
On the other hand, a requirement exists to reduce the weight and size of single-lens reflux cameras. In order to meet this requirement, recent camera bodies are smaller in size. However, this means that the space available for installing electronic components in the camera is reduced and accordingly it is now necessary to install a more intricate circuit in to a smaller space. Accordingly, the wiring on the printed circuit board is necessarily more complicated. However, it is obviously not desirable to lower the work efficiency and reliability of the camera, hence an important problem now exists to technically and simply install an intricate circuit in a limited space in the camera.
One of the flexible printed circuit boards used now is a flexible board having a copper foil on its one surface (hereinafter referred to as "a single-surface flexible board" when applicable). Since the single-surface flexible board has the copper foil on only one surface, the wiring is provided on only one surface of the flexible board. Therefore, the wiring cannot be done unless the flexible board is in planar form. Accordingly, sometimes it is necessary to achieve the wiring with lead wires or jumper wires. However, this is not desirable, because if the number of lead wires is increased, the work effeciency and reliability are lowered.
Furthermore, since the components are installed only on one surface of the printed circuit board with exception of the case where through-holes are formed therein for mounting discrete components, the use of that type of printed circuit board is not advantageous in the utilization of the limited space.
On the other hand, a flexible board with copper foil layer on its both surfaces (hereinafter referred to as "a double-surface flexible board" when applicable) is known, and is suitable for reducing the number of lead wires. However, this type of flexible board is still disadvantageous in that through-holes must be formed therein for wiring. In order to positively achieve the through-hole plating, the diameter of the through-hole should be approximately 0.8 mm and the diameter of the land should be approximately 12 mm; that is, the through-holes occupy a relatively large space on the double-surface flexible board. Accordingly, the formation of the through-holes in the flexible board is to a certain extent necessarily limited. As the circuit becomes more intricate, the number of conductors is increased, and therefore, sometimes it is impossible to form the through-holes in the flexible board. For instance, a typical case is where adjacent ten pins of a small flat package type integrated circuit (IC) with about fifty pins spaced at pin intervals of 0.65 mm are extended through through-holes in the flexible board to the opposite surface thereof. If in this case the through-holes are aligned longitudinally at land intervals or 0.5 mm, then it is necessary to use a space 16.5.times.1.2 mm. This area is excessive when compared to the available space in a camera body. Thus, it is not practical to provide through-holes occupying such a large space of the printed circuit board which is to be installed in a small space in the camera body.
As is apparent from the above description, both the single-surface and double surface flexible boards still have disadvantages for use as an installed electronic circuit in a camera body.
Accordingly, an object of this invention is to provide an excellent printed circuit board in which two single-surface flexible boards are piled one on another in such a manner that their copper foil layers face in the same direction.
It is another object of this invention to eliminate the drawbacks accompanying the above-described single-surface and double-surface flexible boards.
Yet another object of this invention is to provide for a circuit board system useable in cameras that has high reliability yet does not require more than the limited space available in a modern camera body.
These and other objects of this invention are accomplished by means of using at least two flexible printed circuit boards with electrical conductive layers piled on top of each other. The conductive layers face in the some direction and a portion of the lower flexible printed circuit board is left uncovered. Hence the lead wires to some components can be coupled to the electrical conductive layer of the upper flexible printed circuit board. The remaining leads can be connected to the conductive layer on the lower flexible board.
Also, the portion of the lower flexible board not covered by the upper board is raised so that the copper foil layer on the upper board is substantially flush with the corresponding surface on the lower board.
This invention will now be described with reference to the drawings and the description of the preferred embodiment.